1. Field of the Invention
The present invention relates to heat resistive phosphate sintered bodies and a process for producing the same. More particularly, the invention relates to heat resistive phosphate sintered bodies which have excellent heat cycling stability while their dimensional changes after heat cycling are not more than 1%, although the sintered bodies contain microcracks. The invention also relates to a process for producing such heat resistive phosphate sintered bodies.
2. Related Art Statement
Recently, demands for materials having excellent heat resistance and low expansion have been increasing with advance in industrial technologies.
Among such demands, it has been made clear that zirconyl phosphate (ZrO).sub.2 P.sub.2 O.sub.7 is promising as a material having excellent heat resistance and low expansion.
Further, zirconyl phosphates of alkali metals such as sodium have recently been proposed as materials having good heat resistance and low coefficients of thermal expansion (Mat. Res. Bull., Vol. 19, pp 1451-1456 (1984), Journal of Materials Science 16, 1633-1642 (1981) and Yogyo Association Report, No. 95 [5], pp. 531-537 (1987)).
Further, phosphate compounds of alkaline earth metals, which have specific compositions, have been proposed as compounds having low expansion [Mat. Res. Bull., Vol. 20, pp. 99-106, 1985, J. Am. Ceram. Soc., 70 [10] C232-C236 (1987), and U.S. Pat. No. 4,801,566].
U.S. Pat. No. 4,675,302 proposes ceramic materials having a fundamental composition of Ca.sub.0.5 Ti.sub.2 P.sub.3 O.sub.12 as having low expansion.
Further, trials have been effected to combine a specific phosphate compound with zircon to obtain a composite material (Journal of Materials Science, Vol. 20, 4617-4623 (1985) and EP-0 260 893A2.
In addition, EP-0 30 6242 A2 discloses a solid solution of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24, sintered bodies containing such a solid solution, and a process for producing the same.
However, although the phosphate compound such as zirconyl phosphate advantageously has low expansion, such a phosphate is thermally decomposed at high temperatures of not less than 1,200.degree. C., and a phosphor component (P) evaporates. For example, when thermally treated at 1,400.degree. C. for 100 hours, zirconyl phosphate and sodium zirconyl phosphate exhibit as much as 19% and 36% in weight reduction, respectively.
Further, ceramic materials proposed in U.S. Pat. No. 4,675,302 are mainly used as substrates for low expansion optical reflectors which are free from deformations due to temperature changes and which have low expansion. Those optical reflectors are to be used in artificial satellite. As shown in an FIG. 2, it is presumed that the temperature change is considered to be up to around 500.degree. C. at the maximum. However, no attention is paid upon heat stability and wear resistance at high temperatures of not less than, for example, 1,200.degree. C.
Incidentally, as a process for producing phosphate compounds, a process is known, which uses Na.sub.2 CO.sub.3, ZrO.sub.2, ZrOCl.sub.2 .multidot.8H.sub.2 O, SiO.sub.2, (NH.sub.2).sub.2 HPO.sub.4, H.sub.3 PO.sub.4, Nb.sub.2 O.sub.5, Y.sub.2 O.sub.3, SrCO.sub.3, K.sub.2 CO.sub.3, CaCO.sub.3 and the like in combination [T. Oota and I. Yamai, Journal of the American Ceramic Society, 69, 1 (1986)].
However, since a P.sub.2 O.sub.5 component is singly produced in a decomposing step of ammonium phosphate or H.sub.3 PO.sub.4 in this process, the concentration of phosphorus locally becomes higher so that a compound having a low melting point is formed during sintering. Consequently, great pores (voids) are formed around the low melting point compound in the sintered body to cause grave defects.
Further, sintered bodies of specific phosphates suffer dimensional increase or decreased strength in a heat cycling deterioration test by repeatedly heating and cooling for instance between 100.degree. C. to 1,260.degree. C. As the case may be, the sintered bodies may be broken.